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So since Na2CO3 is the solute, and they give you the grams, just convert to moles:

(5.3g Na2CO3)(1 mol Na2CO3/105.99g Na2CO3)

= 0.0500 mol Na2CO3

Also, the solution has to be in liters, so convert the mL to liters:

(400.0 mL)(1 L/1000mL)

= 0.400 L

Now that you have all of the information, just put it all together into the formula:

Molarity = moles of solute ÷ liters of solution

Molarity = (0.0500 mol)/(0.400 L)

Molarity = 0.125 mol/L or 0.125 M